In recent years, a power transmission shaft, for example a propeller shaft which is installed on a vehicle, is equipped at its axial front end with a tubular member and at its axial rear end with a shaft member. The tubular member is connected to the side of a transmission of the vehicle, whereas the shaft member is connected to the side of a differential gear. The rear end of the tubular member and the front end of the shaft member are engaged with each other so as to be relatively movable in the opposite axial directions.
On one hand, the tubular member has a female spline joint portion (an internally-cut, longitudinally-extending keyways or internal splines) formed on the inner peripheral surface of its rear end. On the other hand, the shaft member has a male spline joint portion (an externally-cut, longitudinally-extending keyways or external splines) formed on the outer peripheral surface of its front end. The female spline joint portion of the tubular member and the male spline joint portion of the shaft member are axially engaged with each other by spline-connection, so as to permit axial movement of the tubular member relative to the shaft member. Thus, when an excessive input load is applied from the transmission to the tubular member, for instance, during a vehicle collision, an impact can be absorbed and reduced by virtue of relative movement of the tubular member toward the shaft member.
Also interposed between the rear end of the tubular member and the front end of the shaft member is a stepped boot having large- and small-diameter portions. The boot with the large- and small-diameter portions is made of synthetic rubber and integrally formed into a stepped cylindrical shape. The large-diameter portion (one axial end) of the boot is fixed or retained onto the outer peripheral surface of the rear end of the tubular member by means of a boot band. The small-diameter portion (the other axial end) of the boot is fixed or retained to the outer peripheral surface of the front end of the shaft member by means of a boot band. Hence, the stepped rubber boot, which is fixed in a manner so as to cover the circumference of the spline-connected section of the rear end of the tubular member and the front end of the shaft member, provides a seal around the spline-connected section of the tubular member and the shaft member. One such power transmission shaft has been disclosed in Patent document 1.